If going to Europe, try to avoid going in July and August as was mentioned. Not only are crowds packed in Med, even if you go to other areas in Europe you may see some businesses closed because the owners went on holiday.
We did Paris for Spring Break and we all had a great time (kids 6 + 8). I expected to be disappointed and it was very good. I hate big cities and only acquiesced because it was my daughter's wish list location.
I would recommend Croatia to any and everyone. Nice combination of natural beauty, cities, history and beach/water. Fantastic and welcoming people is an added cherry on the top.
I have also done Scotland and Ireland. I can't recommend them enough. Scotland is gorgeous with natural beauty everywhere. Ireland is more developed but also great people everywhere.
In terms of transportation, I tend to get a car wherever I go because I like to be on my own schedule and visit smaller, local cities. Never had my issues during in Europe, no matter which side of the road.